Synchronous motors with permanent magnets, such as stepping motors, hybrid motors, or direct current motors with no commutating element, are currently well known and used to replace direct current motors with a commutating element, the latter having a relatively short lifetime because of friction generated on the commutating element by carbon brushes.
In motors with permanent magnets, an electronic phase switching circuit is necessary to replace the commutating element function. In synchronous motors, the rotor speed is the same as that of the rotating stator field. When the commutating element is removed and replaced with an electronic circuit, it is necessary to determine the position of the rotor for the control logic to be able to perform the switching at the right time. This is usually achieved with Hall effect probes or optical sensors, which are also called direct sensors.
This type of direct sensors has some drawbacks. First, their costs have a non-negligible impact on the whole cost of the motor. This problem can be partially solved by using a low resolution position sensor as described in U.S. Pat. No. 6,653,829. In this case, however, a state filter must be associated with the sensor to compensate for its low resolution. Secondly, space has to be especially provided not just for the sensors themselves, but also for the related electric connecting means. Therefore, the assembly of such motors is much more complex and time consuming. Finally, the reliability of the system is reduced.
Some existing systems propose to overcome these drawbacks and provide a method and/or a device for controlling a synchronous motor with a permanent magnet with no direct sensors. This is, for example, disclosed in U.S. Pat. No. 6,326,760 or No. 6,005,364, which describe a method and a device to determine the speed of the motor by measuring the induced voltages, in at least two phases, when the driving power in said phases is turned off. However, such a method suffers from the following main drawback. Since the motor with variable load cannot be reliably started by means of a closed loop working in function of the position, this method requires an open-loop starting algorithm for the motor to reach a speed level that is high enough to: 1) create a motion with sufficient kinetic energy to prevent the motor from being stopped by the load between two steps of the control algorithm, and 2) generate induced voltages with a sufficiently high magnitude to allow the rotor position to be determined and thus, the motor to be speed and/or torque controlled.